JPEG image processing is irreversible.
Al Fasoldt's reviews and commentaries, continuously available online since 1983

T e c h n o f i l e
Sample photos show how JPEG damages images

August 6, 2006

By Al Fasoldt
Copyright © 2006, Al Fasoldt
Copyright © 2006, The Post-Standard

   Most digital cameras designed for consumers have one big flaw. I doubt many casual photographers know about it.
   The flaw? The pictures taken by most consumer-level cameras (non-pro models, in other words) are stored in the camera's memory as JPEG images. JPEG (also spelled JPG, and always pronounced "jay-peg") is an old method of stripping details out of digital photos so that they take up less space. A JPEG image, even if it is set to maximum quality mode, is always missing some of the content of the original scene.
   What's more, JPEG processing causes double trouble. First, JPEG image processing is irreversible. Once your photo has been put through the JPEG grinder -- once that sirloin has been turned into hamburger -- there's no way to get back to the original steak ... er, photo. Second, repeatedly applying JPEG image processing makes matters worse. Saving a JPEG image multiple times after editing it forces your software to crunch the photo's details again and again. The picture inevitably suffers.
   I've written about this problem many times (see www.technofileonline.com/texts/tec092301.html for an overview), but I've never tried to show the kind of damage JPEG processing causes. This week, I'm letting the pictures tell the story.
   But before I show you the evidence of JPEG improprieties, I'll explain how to deal with the JPEG conundrum. Follow these three rules:
   1. Always set your camera to take the highest quality photos. You might have two settings (one for resolution and another for "quality"). Resolution should always be at the highest numbers and "quality" should be as high as possible. ("Quality" in this case represents how sparingly the camera removes details when it creates JPEGs. The higher the "quality" setting, the less the processing.)
   2. Always make non-JPEG copies of your pictures as soon as you get them out of your camera. The non-JPEG versions must be any of the following types: TIFF (also spelled TIF), PSD, PNG or BMP. Use the non-JPEG copies from that point on for everything. Never edit or resave the JPEG versions; store them as backups only.
   3. Make your non-JPEG copies using software that will work on many photos at once. Don't do it manually, one at a time. Irfan View does this nicely in Windows (www.irfanview.com) and iPhoto does it (open the File menu, then click Export) on an OS X Macintosh. 
   All the images here are PNGs. The PNG format, unlike JPEG, is lossless, and thus it won't interfere with what I am trying to show you.
   Now for the photos. I grabbed a publicity photo of a wireless base station from Apple Computer -- a TIFF without any image compression -- and saved a copy of it as a JPEG. (The image you're seeing above is a PNG copy of this photo, made very small, just to show you what the base station looks like; the TIF image is huge.)
   What you are seeing in the photos below are the comparisons -- using extreme enlargements -- of a portion of the original basestation photo and the same portion after it was saved as a JPEG.
   I used iView Media Pro's "Light Table" to compare the two versions close up, showing various details. You're seeing the comparisons below as captured from iView Media Pro.
   In each case, the JPEG copy was made using a high compression/low quality setting to emphasize the differences. Remember that JPEG processing always damages the image in certain ways; the comparisons here are more pronounced to make them more noticeable. (There is no such thing as a "lossless JPEG" image. Read today's blog for more on this issue.)
   The original photo is at left in each shot, and the JPEG copy is at right.

   In the photo directly above, notice how the ellipsoid shape looks blocky in the JPEG version and how the slope below it looks fairly smooth in the original but badly mottled in the JPEG.
   In the photo below, you see part of the word "AirPort." Notice how the JPEG version fuzzes up the characters and leaves faint block-like appendages. JPEG processing is particularly unkind to the written word.